I changed the 12-25 8sp cassette on my bike to an 11-30 as it better
matches my new commute. The rear derailleur is a Tiagra of 2006
vintage. I believe the model is a RD-4400-GS. I adjusted the b-screw
so that the jockey wheel just barely clears the rear sprocket in the
42:30 gear. The setup shifts well in the low and high cogs but shifts
slowly in the middle - it has a lot of trouble shifting from 17 to 20
and from 20 to 23, requiring a double downshift then upshift
(otherwise it makes a bunch of noise and the chain skips a lot but it
doesn't catch on the larger sprocket.)

Is there another adjustment I can make to get the jockey pulley to
ride closer over the middle cogs? Or will I be better served by an MTB
derailleur?

-pm